Correct Featured Image Size on Modularity Lite

I've been using featured images for a while and everything seems to be working fine. These images appear on Facebook and Twitter and there is no problem at all.
But one day I decided to try to use the search bar of my own blog. The results were horrible. I was looking for a page that included a featured image, but it was way too big! It didn't fit there, so half of it wasn't even visible (look at the image below).
Now I want to re-upload all my pictures at a correct size, so that they can be featured images, without being way too big or way too small. But...what size should they be?
P.S. I use the Modularity Lite Theme.
P.S.2 I hope you can see this image: http://gunsnwar.wordpress.com/?attachment_id=2336

Oh, I'm afraid that you misunderstood me. My bad...
I'm not talking about this kind of featured images. I'm talking about what kind of size should the featured image of posts and pages should be, so that when someone uses the search bar of my blog, they will appear at their full size, without any problem.